An engine having an exhaust after treatment system may produce its highest concentration of regulated tailpipe exhaust emissions during engine starting when the after treatments system may be operating at a lower level of efficiency. One way to reduce tailpipe emissions is to rapidly heat the exhaust after treatment system so that the exhaust after treatment system may more efficiently convert regulated gases. The exhaust after treatment system may be heated more rapidly via increasing an exhaust mass flow rate to the after treatment system. However, increasing the exhaust gas flow rate beyond what is required for rapid catalyst heating may increase fuel consumption more than is desired. Further, increasing the exhaust mass flow rate may require increasing the engine air mass flow, and increasing the engine air mass flow may interfere with providing vacuum to operate vacuum actuators. In addition, when external loads are applied to the engine, the engine air flow rate (and thus exhaust mass flow rate) may be further increased to compensate for the increased engine load. Consequently, engine exhaust heating rate may be increased as engine load increases while there may be no need or desire to provide higher engine exhaust heating rate.
The inventors herein have recognized the above-mentioned disadvantages of engine starting and have developed an engine starting method, comprising: providing a substantially constant air mass to the engine after engine run-up until a predetermined condition; retarding spark timing from MBT timing to provide a torque to achieve a desired engine idle speed while the engine inducts the substantially constant air amount; and advancing spark in response to an increase in engine load while the engine inducts the substantially constant air mass.
By providing a substantially constant air mass to an engine after run-up, it may be possible to limit unnecessary fuel consumption and elevated exhaust gas temperatures that may not be necessary or desirable during some engine operating conditions. Further, it may be possible to provide a desired level of vacuum even during changes in engine load when the engine operates at a substantially constant air mass flow rate. For example, an engine air mass flow rate that provides a desired level of heat flux may be commanded via adjusting throttle position or valve timing. The engine air mass flow rate may be selected such that the engine provides a desired level of vacuum when operated at the selected engine air mass flow rate. Additionally, since the engine air mass flow rate is held constant during the engine/exhaust after treatment heating period, additional unrequested heat is not provided by the engine to the exhaust system when limited engine load changes occur.
The present description may provide several advantages. Specifically, the approach may reduce unnecessary fuel consumption during engine heating. Further, the approach may allow an engine to provide more uniform vacuum for vacuum consumers during starting. Further still, engine emissions may be reduced from the engine because a substantially constant air flow through the engine allows for substantially constant fueling and a reduction of the possibility of air-fuel errors during engine starting when conversion efficiency of exhaust after treatment devices may be low.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.